In 2008, the United States produced 8.5 million barrels of petroleum oil per day while the demand was 19.5 million barrels per day, thus only half of the requirement could be produced domestically. Alternative sources like ethanol can play a vital role in this situation. Presently, about 9 billion gallons of ethanol is produced per year, which is only a quarter of the amount of renewable fuel that the U.S. is looking for by 2022 according to The Energy Independence and Security Act (EISA) of 2007. Some potential sources like corn are not feasible because of the deficiency of food in the world. Again, with the environmental problems caused by various organic wastes, lignocellulosic biomass is considered one of the most important sources of renewable energy and, of course, it is not a food. Coniferous and deciduous wood, switch grass, rice hulls, corn stover, straws, and bagasse are typical types of lignocellulosic biomass. Direct combustion of lignocellulosic biomass can produce energy but handling and transportation are of concern in commercializing this process despite the fact that lignocellulosic biomass itself is not expensive. With thermochemical pretreatment, biomass can be made more dense in both mass and energy; reducing transportation and handling difficulties. Moreover, it reduces hydrophillic behavior making it simpler to store, and, also increases grindability.
Wet torrefaction and dry torrefaction are two promising kinds of pretreatment processes. Biomass treated with hot compressed water in an inert atmosphere is known as wet torrefaction or hydrothermal pretreatment, while dry torrefaction does not use water. Temperatures of 200-260° C. are needed for wet torrefaction and pressures reach up to 700 psi. The solid product has 55-90% of the mass and 80-95% of the fuel value of the original biomass. For dry torrefaction, the temperature is maintained at 200-300° C., and it can recover 60-80% of mass and 70-90% energy value. In earlier attempts at wet torrefaction reaction, the reaction temperature was slowly achieved over 10-15 minutes before the desired reaction temperature was achieved and maintained for at least 5 minutes and more commonly for several hours. For dry torrefaction, the reaction temperature is maintained for about 80 minutes.
Long reaction times used in the older processes require greater energy input and larger reaction vessels thereby decreasing the net energy yield from the reacted biomass. As such, processes and devices that increase the efficiency of wet torrefaction process are needed so as to improve the energy yields and to meet the growing energy demand.
Moreover, the long reaction times used in older processes result in the formation of a byproduct sludge that increases the difficulty of processing and handling biomass treated with those processes. Processes and devices are needed that increase both mass and energy density of biomass under conditions that either do not form the byproduct sludge or that form substantially reduced levels of the sludge.
Transporting, handling, and storing untreated lignocellulosic biomass can be challenging because of the low bulk density of the untreated biomass and the tendency of the biomass to rot unless stored under ideal conditions. Treatment of lignocellulosic biomass, for example, with dry torrefaction or wet torrefaction processes that utilize long reaction times can improve the bulk density and increase the range of storage conditions necessary to prevent rotting, but the treated products themselves are frangible and can be very difficult to handle. Forming the treated lignocellulosic biomass into pellets can remedy these problems. Forming pellets from treated biomass requires the presence of a binder in the biomass to maintain the structural integrity of the pellets. However, biomass treated under the dry torrefaction conditions or wet torrefaction processes that utilize long reaction times, destroys completely, or degrades a large portion of the natural materials found in the biomass that could function as a binder, namely lignin. Processes that increase the bulk density of lignocellulosic biomass while preserving sufficient levels of natural binding agents are needed.